1. Field of the Invention
The present invention relates to the polymerization of ethylenically unsaturated (co)monomers in the presence of certain conjugated diene chain-transfer agents (or chain-limiting agents).
2. Description of the Prior Art
To restrict or limit molecular weight and/or the degree of crosslinking of the ultimate polymers, conventional polymerizations require use of a chain-transfer agent (or chain-limiting agent), which elicits its intended effect during the polymerization process. The transfer agents currently used industrially include aliphatic mercaptans and halogenated hydrocarbons. Advantages are associated with each of these two types of regulating agents. Although they have a very low residual mercaptan content, the polymers produced in the presence of a mercaptan typically possess an undesirable odor. If organohalogenated compounds are used, such as carbon tetrachloride, bromoform, or bromotrichloromethane (see, for example, U.S. Pat. No. 4,176,219), the residual amounts thereof are relatively high.
U.S. Pat. No. 3,726,832 describes the preparation of polymers of conjugated diolefins having a molecular weight of less than one million, at a high polymerization rate, and using cyclic ethers as the agent regulating the molecular weight.
WO-A-88/04,304 describes a chain-transfer agent selected from among compounds corresponding to the formulae: ##STR2## wherein R.sub.1 is preferably a radical, such as an aromatic radical capable of activating the ethylenic double bond; R.sub.2 is, in particular, an alkyl radical, and X is a heteroatom.
According to such '304 patent, the chain-transfer agents having the formulae (I) and (II) possess a chain-transfer constant of approximately 1, thereby enabling production of polymers having a narrow molecular weight distribution.